If you spot two enormous birds locked
together in what appears to be a soaring, tumbling battle for dominance of the
sky, then look again! You're most probably watching a pair of wedge-tailed
eagles re-igniting their passion prior to breeding.
By Rachel Sullivan
Wedge-tail eagles are among
the world's largest eagles with a wingspan of up to 2.5 metres. They are
relatively common all over the Australian mainland, in Tasmania and southern New
Guinea, except in densely populated areas.
To get off the ground they use an active flapping
flight, but once airborne these eagles use thermal updrafts to carry them
effortlessly aloft and gracefully spiral upwards to great heights.
Australia has three species of eagle, the Little
Eagle (Hieraetus morphnoides), the White-bellied Sea Eagle
(Haliaeetus leucogaster) and the largest, the Wedge-tailed Eagle
(Aquila audax - literally 'bold eagle'). Wedge-tails prefer open
woodland, but are found exploiting a range of habitats from arid desert to
grasslands, mountainous areas, and even rainforest.
Easily identified by their size (up to 3.2 kg for
males and 4.2kg for females), Wedge-tails are known as booted or trousered
eagles thanks to the heavy feather 'trousers' covering their legs. They are also
characterised by finger-like wing feathers and, of course, a long wedge-shaped
tail. Young birds can be distinguished by their light brown plumage and golden
highlights; the plumage of older birds tends to darken to near black at sexual
maturity between five and seven years.
Fiery courtship
According to Dr Penny Olsen, a research scientist and
raptor expert from the ANU, Wedge-tails appear to be monogamous, mating for
life. "They certainly form long-term pair bonds," she says, "although if one
bird is killed the survivor will take a new mate."
The time of breeding depends on temperature to some
extent. In Tasmania, Wedge-tails breed later than their northern counterparts,
but all breed between June and October.
"Breeding season is the best time to see Wedge-tails
because of their highly visible soaring and diving displays," says Olsen. Males
may also mark their territorial boundaries by flying in a series of showy
descending loops, or 'pothooks'.
In another dramatic manoeuvre known as talon
grappling, two birds lock talons with one being forced to flip over and fly
upside-down, which although spectacular can be fatal if one bird refuses to let
go. Olsen says the research community is divided on whether this is an
aggressive display aimed at warning off potential interlopers, or just two birds
showing off as part of their courtship ritual.
Position, position
Breeding pairs establish territories which vary in
size depending on the abundance of prey. These territories are used throughout
the year, and within them the eagles may build several nests, some of which are
used as feeding platforms.
Seeking a clear view over a wide area, they generally
build their nest in the largest tree available. And it needs to be big: while a
nest can start as small as one metre in diameter and just 75cm deep, as sticks
are added over successive years, the structures can grow up to four metres deep
and two metres in diameter, and weigh in at around 400kg.
At laying time a shallow depression in the centre of
the nest is lined with fresh leaves, and between one and three eggs (usually
two) are laid at two-four day intervals. The male and female take turns to
incubate the clutch for approximately 45-47 days, and then the eggs hatch one at
a time.
Fact file:
When: June to
October
Where: Wedge-tailed eagles are
found throughout the Australian mainland and Tasmania.
Nestling skulduggery
During the first five weeks adults place food
directly into the chicks' mouths, but from then onwards the chicks can recognise
and pick up food from the nest floor. The parents also protect their young from
predators and the elements; if threatened, larger chicks lie flat in the nest,
but will put up a spirited defence if necessary.
However, the greatest threat to a chick's survival is
not predation, but its oldest sibling: although survival rates vary considerably
depending on local conditions usually only the strongest chick survives. In a
good year, a pair of chicks may be fledged, but when times are tough, the oldest
chick will kill and eat its nestmates.
Once fledged, the young eagles remain with their
parents for up to three months before gradually dispersing up to several hundred
kilometres away. If they survive their first year (and two-thirds don't),
wedge-tailed eagles can live for at least 40 years in the wild and more than 60
in captivity.
Past misunderstandings
The birds' large size means that they can take a
whole range of prey from small lizards to ground-foraging birds, possums,
wallabies and kangaroos. Lacking the speed of other raptors, Wedge-tails are
designed for intercepting prey on the ground, and will even cooperate with other
birds to hunt large prey; Olsen says several eagles will cooperate to bring down
a large kangaroo.
On the whole, the birds have benefited from European
arrival in Australia - woodlands have been opened up for pasture and many
introduced species, including cats, foxes, and particularly rabbits, are the
perfect sized prey for a hungry eagle.
However, while they actively hunt live prey during
the breeding season, Wedge-tails are enthusiastic carrion eaters, and have
frequently been observed eating recently deceased lambs. For many years angry
farmers believed the birds were picking off their young stock, and more than
30,000 birds were killed in the 1970s for what research has now revealed a major
misunderstanding.
These days it is recognised that few Wedge-tails
actively hunt lambs and calves; ironically most are actually doing the farmers a
service by rapidly consuming any dead animals and so minimising the chance for
flies to breed. There are still occasional 'misunderstandings' between farmers
and eagles, but they are now protected in all states.
Rabbits and Windfarms
While they lack natural predators a couple of new
threats to Wedge-tail survival have emerged in the past decade or so: rabbit
Calici-virus and the growth of windfarms in eagle hunting grounds.
Like other animals that relied heavily on rabbits
Wedge-tailed Eagles experienced some local population declines in the aftermath
of the Calici-virus outbreak. But although the virtual disappearance of their
favourite prey caused serious short-term consequences for some rabbit-dependent
populations, Wedge-tails are such generalist predators that they simply switched
to eating other things and overall numbers have not suffered.
Windfarms have also caused concern among
environmentalists for years, who fear that birds may blunder into the slashing
turbines. Dr Olsen says that occasionally Wedge-tails have met their end that
way.
"Normally, they know where the turbines are and avoid
them," she says. "However accidents tend to happen - including two recent
casualties in Tasmania--when it is foggy or there are high winds that interfere
with the birds' normal navigational abilities."
Most governments are aware of the problem and try to
minimise the impact of any new developments on the local bird life. "New
turbines in development, in which the fan sits horizontally, might also help
reduce their impact," says Olsen.
Credits
Special thanks to Dr Penny Olsen and
Peter Merritt.
http://www.abc.net.au/science/articles/2006/07/06/2041996.htm
